How does a Fusion drive appear to the system

macrumors 6502

A lot of hardware RAID implementations appear under Disk Utility as nothing more than regular hard drives. If you use the command line version of Disk Utility using the command line version "diskutil" it will often do as follows:

(that's a fake list I made up, FWIW but in the above disk1 could be the way a lot of hardware RAID units appear to the system - just as a normal drive even though the RAID unit may have 4 drives in it.

There's a lot of post on the web with titles like "build your own Fusion drive." It uses CoreStorage to set up some drives/volumes into a combined unit.

I'm curious. If someone does a "diskutil list" on a REAL fusion drive, will all the CoreStorage elements show up, or does it, like a RAID unit, simply appear to the OS as just a regular drive.

FWIW, if any of you guys have tried spanning drives with CoreStorage, "diskutil list" will output an entry for each CoreStorage entry and then a final one for the complete, logical volume. In other words, if you used CoreStorage to span 3 disks and make it appear like one big drive, you end up with 3 CoreStorage components and a final usable volume, thus it looks like you have 4 drives on your system.

I'm just curious, because if a Fusion drive appears as a single drive under disk utility, it implies that there's hardware doing a little more than what the guys promoting the "build your own Fusion drive" are saying....or so I think. On the other hand, if each drive inside a real Fusion drive show up as individual drives and volumes under diskutil, then it would kind of imply the actual Fusion drive is nothing more than a glorified SATA extender with an SSD and hard drive installed in it.

thread startermacrumors 6502

It may not be surprising but from a support standpoint the lack of the Thunderbolt identifier could be a PIA.

Consider the following: Someone has bought a Thunderbolt external drive (not fusion) and something has gone wrong. He used to have 2 disks, one internal and one external, and one of them has blown. Unless that user is aware of what the sizes of the drives are or what designation they had, I doubt it would be immediately obvious to them which one has failed. If I asked them to open up a terminal and type "diskutil list" only one drive will show up and, regardless of whether or not it's internal of Thunderbolt, it will appear as disk0.

If I ask them to open up System Information, will the drive be listed under "Thunderbolt" or will it be listed under "Serial-ATA?" (I don't know the answer to that question). If the external drive shows up under "Serial-ATA" then there's really no way to help someone from a remote location with this problem.

Apple clearly could have done a better job designating these things. They could have ID'd the drive as something like "SATA under Thunderbolt" or "FireWire under Thunderbolt" (for a FireWire drive).

I have to wonder if this isn't why some of the guys developing drive testing software seem to be getting to the game a little late. Just during the last week, Scannerz (finally) added full support for apparently all forms of CoreStorage. The TechTool Pro site STILL says that to test an encrypted drive you need to do it with e-Drive, which is telling me it's seen as an unidentified drive type, and apparently they're still testing Fusion Drives (probably because CoreStorage disk management might mess up their ability to do index checking - a guess for sure on my part!)

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